Samir Laoui, Ph.D. 3/09/2016

Scope & Purpose

AAPM Task Group Report 106 has been prepared

to

“ facilitate accelerator beam data commissioning

by describing specific setup and measurement

techniques, reviewing different types of radiation

phantoms and detectors, discussing possible

sources of error, and recommending procedures

for acquiring specific photon and electron beam

parameters. ”

Introduction

The data obtained during commissioning is

treated as the standard for clinical use and should

be verified periodically

At minimum, the following should be collected

during commissioning:

Photons: PDD, profiles (in and/or cross plane) at

various depths for open and wedged fields, data related

to MLC… etc.

Electrons: PDD, profiles, cone factors, insert factors,

and virtual source positions

Issues

Data can vary with the choice of phantoms and

detectors and methods used

Data

Phantoms, methods and detectors

Phantoms Scanned data must be carried out with a water

phantom (40x40x40 min). Point dose may be

carried out in solid water phantom

Scanning in both in-planes and cross planes

Distilled water, with algae growth prevention

Room temperature

Evaporation/Surface should be verified

75x75 cm2 is optimum recommended size

Scanning system setup

Water tank needs to be calibrated

Check the free movement of each arm, and the x, y , z,

and diagonal motion

Accuracy and linearity, leakage, cracks, mechanical

stability, cables quality.

Phantoms, methods and detectors

Detectors Ion chambers: Can provide direct dose measurement.

Cylindrical, spherical and parallel plate

Diodes: Energy independence of mass collision stopping

powers (Silicon to Water). Attractive for electron dosimetry

Detector array: Profile measurements.

TLD: Point dose measurements. Strong energy dependence

and nonlinear dose response

Film: silver halide and Gafchromic. planar dose maps in small

fields measurements.

Scanning and reference detectors

High voltage requirement: Ion Chambers (300-

400 V), diodes: 0 V

Polarity: Agreement between positive and negative

Connections and Cables Most cables used in radiation dosimetry and with

the scanning system have triaxial adapter

Some manufacturers market unusual looking

triaxial ends nonstandard that may not fit standard

ion chambers. PTW is one such vendor that has

different triaxial adapter ends

Scanning Water Phantom

UCI MC makes use of original Wellhöfer

scanning water phantom

Scanning Water Phantom

The scanning tanks should never be placed on

the machine treatment table (weight)

The tank origin should be close to the machine

isocenter

The detector should be level with the water

surface in all four corners of the tank

The z-direction movement of the detector

should be parallel and should follow the

central axis of the machine at 0° gantry angle

Positioning

The detector position should be set such that the center

of the detector splits the water surface: 0 position

Arm tilt, gantry tilt

Premeasurement test

Position the scanning detector at isocenter:

Perform an in-air scan of a 20x20 cm2 field,

allowing the scan to run from −20 to +20 cm (40

cm total). Also perform a scan with beam off

Analyze for the following:

Noise, STNR, response time of the system, leakage,

polarity

Saturation test: Repeat dry run with 20x20 field at

max dose rate

Data Acquisition

Data acquisition should be conducted in an organized

fashion to avoid confusion

Data acquisition speed: critical for small fields

Sampling time should be long enough

Photon beam data

PDD 100 SSD, long tradition of such setup

Scaling of data taken from a different SSD should only

be used as QA checks to ensure consistency for the

following reasons:

Electron contamination: Depend on SSD

Primary dose: can be scaled for different SSDs just by

applying the inverse square law, except for small field sizes

Scatter dose: SSD dependent

Penumbra: It cannot be scaled from one SSD to another

Photon beam data

Beam Profiles The choice of detector orientation is critical for

profile measurements for small fields and high

gradient regions

Small volume detector is preferred for profile

measurements

Beam profiles: Data collection

Maximum of 1 mm spacing in the penumbra region and

preferably no more than 2 mm spacing in the remainder of the

field\

Profiles at 5–7 depths are sufficient for each 1 cm spaced field

size up to 6x6 cm2, and then 5 cm spacing for field sizes

10x10 cm2 and greater is sufficient

Beam profiles

Star pattern: Some TPS require BP with respect to the

collimator angle

Physical wedge: Data collection at smaller spacing in the high

gradient area

Electronic wedge: Standard scanning cannot be used. Profiler

may be used.

MLC Data

These parameters, as described below, should be quantified for

each photon energy and a minimum of four gantry angles (0°,

90°, 180°, 270°) to examine the effect of gravity on leaf

motion

Light and radiation field congruence

Interleaf leakage (leakage between two leaves)/ Portal imager

Intraleaf leakage (transmission though a leaf)/Portal imager

Penumbra

Photon point dose data

For manual dosimetry calculations, the following should be

collected

Scp :

Large water phantom ensures full lateral buildup

The deepest point of measurement should be at least 10 cm to

ensure full backscatter

In-air output ratio Sc: Ratio of primary collision water KERMA in

free-space, Kp, per monitor unit M between an arbitrary collimator

setting and the reference collimator setting at the same location.

Measured with IC with a buildup cap

Photon point dose data

Sp: is defined as the ratio of the scatter factors between the

actual field size, s, in the phantom and that of the reference

field size, sref Where SF= D/Dprimary

Wedge factors, tray factors

Small field considerations: Small-field dosimetry is

challenging due to lack of lateral electronic equilibrium

Electron Beam

Diode, parallel plate ion chamber, cylindrical ion chamber and

films

Percent depth ionization curves should be scanned for all

energies for the reference cone to a depth of Rp+10 cm with

depth increment of 0.1 cm

When IC is used for measuring depth ionization curves in

water, reading should be converted to dose using stopping

power ratios

Small volume electron diode is ideal since it does not require

ionization to dose conversion

Profiles

Attention should be given at depths greater than dmax,

especially for low energies

Electron point dose data

Cone factors: Ratio of dose at dmax for a given cone to the dose

at dmax for the reference cone

Cutout factors: Ratio of the dose with and without the cutout at

their respective dmax

TrueBeam Biannual QA

Photons:

5 energies * 8 fs = 40 PDD scans

5 energies * 4 depths (dmax, 4, 10, 15) = 20 Diagonals

5 energies * 4 depth (dmax, 4, 10, 15) * (IPxCP)= 40 profiles

Wedge and accessory TF:

3 energies (4 physical, 4 EDW) = 24 measurements

Electrons:

6 energies * 5 cones = 30 PDD/PDI scans

6 energies x 1 cone (15x15) = 12 profiles

TrueBeam Biannual QA- TG 51

5 Photons

TrueBeam Biannual QA- TG 51

6 Electrons

Miscellaneous

MU Linearity

Output constancy Vs. gantry angle

Mechanicals

The end